I have slim body. How to make will fat body if i...

Underweight causes 

• eating disorders such as norexia nervosa, Bolimia nervosa
• having an overactive thyroid 
• food alleries such as gluten intolerance
• medical condition such as diabete, cancer, infections

Tips to gain weight

• You need to eat more calories than your body burns, 300-500 calories per day above your maintenance level, then increase 700-1000 calories.
• eat sufficient protein to gain muscle weight muscle instead of just fat.
• don't drink water before meals, this can fill your stomach and make it harder to get in enough calories.
• eat nuts every day such as almonds, walnuts, macadamia nuts, peanuts
• eat Dried fruit as a snack such raisins, dates, prunes 
• drink whole milk, full-fat yogurt, cheese, cream.
• add health fats and oils such as olive oil and avocado oil to your food
• eat grains4 times per day, whole grains like oats and brown rice.
• eat 2-3 meat per day such as chicken, beef, pork, lamb
• focus on dence food like dark chocolate, avocados, peanut butter, coconut milk, granola, trail mixes.